Preparation of lower tetraalkyl quaternary ammonium nitrates



United- States Patent Delaware No Drawing. Filed Feb. 26, 1959, Ser. No. 795,867

Claims. (Cl. 260--567.6)

This invention relates to a process for the preparation of a quaternary ammonium nitrate and more particularly to the preparation of a lower tetraalkyl quaternary ammonium nitrate.

Lower tetraalkyl quaternary ammonium nitrates have aroused considerable interest in the field of rocket propulsion. These compounds are used primarily as components of liquid rocket fuels, particularly in liquid monopropellants where they contribute to both the oxidizing components and the fuel component. The most common method for the preparation of these compounds consists of reacting a tetraalkyl quaternary ammonium halide with silver nitrate in an aqueous solution. In this process the silver nitrate reacts with the tetraalkyl quaternary ammonium forming tetraalkyl quaternary ammonium nitrate and an insoluble silver halide whichprecipitates out and is separated by filtration. Since such agprocess consumes both silver and the halide, it is costly and thus economically impractical for a large scale operation.

It is, therefore, an object of this invention to provide an efiicient process for the preparation of a lower tetraalkyl quaternary ammonium nitrate. A further object is to provide a process which does not require expensive reactants and is adaptable to a large scale operation.

The above and other objects are attained according to the invention by intermixiug a lower tetraalkyl quaternary ammonium halide with nitric acid in the presence of an unsubstituted branched-chained alkanol having from 4 to 6 carbon atoms. In the presence of the branched chained alkanol, the tetraalkyl ammonium halide reacts with the acid and is converted to the nitrate salt. The hydrogen halide formed by the reaction of the nitric acid and the tetraalkyl quaternary ammonium halide reacts with the alcohol diluent to form water and an alkyl halide.

Lower tetraalkyl quaternary ammonium nitrates having a total of from 4 to 7 carbon atoms and a general formula:

where R R R and R represent an alkyl radical having from 1 to 4 carbon atoms may be thus prepared. Illustrative examples of these compounds, which are finding wide utility as liquid rocket fuels, are tetramethyl ammonium nitrate, trimethylisopropyl ammonium nitrate, and diethyldimethyl ammonium nitrate.

In the preparation of the nitrate a corresponding lower tetraalkyl quaternary ammonium halide is used as a reactant. The halide has a total of from 4 to 7 carbon atoms and a general formula:

R R: Rz Ri where R R R and R represent a lower alkyl radical of from 1 to 4 carbon atoms as described above and X represents a halogen selected from the group consisting of bromine and chlorine. A bromide salt is preferred. These halide reactants may be prepared by known suitable methods, such as by the reaction of a tertiary amine with a monoalkyl halide.

In carrying out the reaction of the tetraalkyl quaternary ammonium halide with the nitric acid, a diluent of a branch-chained alkanol having from 4 to 6 carbon atoms, such as isopropyl, secondary butyl, tertiary butyl, secondary amyl and the like is used. Tertiary butyl alcohol is preferred due to the marked advantage in reactivity obtained when this alcohol is used.

In the presence of the alcohol diluent, the reactants may be added together in any convenient manner and the reaction takes places smoothly. Usually room temperature is suflicient for the reaction, but sometimes the rapidity of the reaction can be increased by heating the reaction mixture slightly. Usually it is preferred to add the tetraalkyl quaternary ammonium halide to the alkanol in such proportions that a solution containing from 10 to 35 percent by Weight of the halide is obtained. About 15 to 25 percent concentration is preferred. After the tetraalkyl quaternary ammonium halide is dissolved in the alcohol, the nitric acid is then added slowly to the solution. At least a stoichiometric amount of nitric acid is required to convert all of the halide to the nitrate form. It has been found advantageous to use a slight stoichiometric excess of the nitric acid, i.e., about 1.1 to 1.2 moles of the acid per mole of the quaternary ammonium halide. The nitric acid most conveniently used is the commercially available concentrated nitric acid which contains around 70 Weight percent of the acid. However, other concentrations of the acid of from about 60 weight percent to weight percent may be used.

The tetraalkyl quaternary ammonium nitrate formed by the reaction of the nitric acid with the halide is insoluble in the reaction mixture and will precipitate out. After the reaction has had time to go essentially to completion the solid product is recovered from the solution by conventional means. For example, the unreacted alkanol, by-product alkyl bromide, and the major portion of the water may be removed by distillation. The residue from such distillation may then be purified by washing with a liquid, such as acetone which is not a solvent for the desired product but a solvent for the other constituents in the reacted mass. The tetraalkyl ammonium nitrate may then be recovered by filtration and dried.

The following examples further illustrate the invention but are not to be construed as limiting it thereto.

Example I In 10 milliliters of tertiary butyl alcohol, 1.8 grams of trimethylisopropyl ammonium bromide were dissolved. To this solution of the tetramethylisopropyl ammonium bromide, 0.6 milliliter of 70 Weight percent nitric acid was added. The mixture was subjected to a mild agitation for about 5 minutes and then distilled to remove a major portion of the liquid from the reacted mass. The distillation residue obtained was slurried with a small quantity of acetone and then filtered. The filtered prodnot was further washed with acetone and vacuum dried to obtain the tetramethylisopropyl ammonium nitrate.

Example II To milliliters of tertiary butyl alcohol, 15.4 grams of tetramethyl ammonium bromide were added. The tetramethyl ammonium bromide was dissolved in the alcohol and then 7.2 milliliters of 70 percent nitric acid was slowly intermixed. The mixture was stirred and warmed gently to about 60 C. After 30 minutes, the reaction mass was subjected to distillation to remove a major portion of the liquid. As the liquid portion of the reaction mass was removed, a precipitate of the product appeared in the residue. After a major portion of the liquid had been removed, the residue was cooled which precipitated more product. The product was recovered by filtration, washed with acetone, and vacuum dried. The tetrarnethyl ammonium nitrate was recovered in an amount of 10.6 grams which was equivalent to a yield of about 80 percent.

Analysis of the product obtained showed that it contained 35.1 percent carbon, 80.8 percent hydrogen and 20.3 percent nitrogen. Theoretical proportions of tetramethyl ammonium nitrate are 35.3 percent carbon, 8.88 percent hydrogen and 20.3 percent nitrogen.

In a manner similar to that described above other tetraalkyl quaternary ammonium chloride or bromide having a total of from 4 to 7 carbon atoms may be reacted with nitric acid and the corresponding tetraalkyl quaternary ammonium nitrate obtained. Also in place of tertiary butyl alcohol other branch-chained alcohols having from 4 to 6 carbon atoms may be used in place of tertiary butyl alcohol.

What is claimed is:

1. A process for the preparation of a lower tetraalkyl quaternary ammonium nitrate, which comprises intermixing a tetraalkyl quaternary ammonium halide having a total of 7 carbon atoms and the general formula:

where R R R and R represent an alkyl radical having from 1 to 4 carbon atoms and X represents a halogen selected from the group consisting of chlorine and bromine with nitric acid in the presence of an unsubstituted branched-chain alkanol having from 4 to 6 carbon atoms to react the tetraalkyl quaternary ammonium halide with the nitric acid thereby to directly precipitate said lower tctraalkyl quaternary ammonium nitrate in the reaction mixture, and recovering the so-precipitated tetraalkyl quaternary ammonium nitrate formed from the reaction mass.

2, A process for the preparation of a lower tetraalkyl formula:

R1 R2 Ra \R4 where R R R and R represent a lower alkyl radical having from 1 to 4 carbon atoms and X represents a halogen selected from the group consisting of chlorine and bromine in an unsubstituted branched-chain alkanol having from 4 to 6 carbon atoms to form a solution containing from 10 to 35 weight percent of the halide, intermixing nitric acid with the solution to react the acid with the tetraalkyl quaternary ammonium halide thereby to directly precipitate said lower tetraalkyl quaternary ammonium nitrate in the reaction mixture, and recovering the so-precipitated tetraalkyl quaternary ammonium nitrate formed by the reaction from the reaction mass.

3. A process according to claim 2 wherein the tetraalkyl quaternary ammonium halide solution contains from 15 to 25 weight percent of the halide and the nitric acid is intermixed in slightly stoichiometric excess as an aqueous solution containing from to weight percent of the acid.

4. A process according to claim 3 wherein the unsubstituted branched-chain alkanol is tertiary butyl alcohol.

5. A process according to claim 4 wherein the tetraalkyl quaternary ammonium halide is trimethylisopropyl ammonium bromide.

References Cited in the file of this patent FOREIGN PATENTS 73 ea Brit i g- 5s 

1. A PROCESS FOR THE PREPARATION OF A LOWER TETRAALKYL QUATERNARY AMMONIUM NITRATE, WHICH COMPRISES INTERMIXING A TETRAALKYL QUATERNARY AMMONIUM HALIDE HAVING A TOTAL OF 7 CARBON ATOMS AND THE GENERAL FORMULA: 